CAPACITORS:

What is a capacitor?

It's a device that can hold charge, they are usually produced by layering a conductor-nonconductor-conductor(In this case, the non-conductor is called a DIELECTRIC) allowing a static electrical field to build up between the two conductor plates.

What are capacitors used for?

Capacitors are ubiquitous and are used in many applications including:- Radios to block particular frequencies/tune them.- Power supplies to 'smooth' the output.